Flash dryers and fluidized bed dryers have been the primary devices known in the past for drying particulates using heated gas.
Here, a flash dryer is a dryer having a structure in which an ascending air current is formed by heated gas in a cylindrical straight pipe, the particulates are fed into the ascending air current, and the particulates are dried while carried aloft by the ascending air current.
Advantages of such flash dryers are that they have a simple structure, and the particulates are dried at the same time that the air is transported, resulting in high treatment capacity.
On the other hand, the ascending air current is formed only in the straight pipe in such flash dryers, resulting in less dispersion of the particulates that are supplied as the material being treated, and when wet particulates or the like which have formed into lumps are treated, a disperser or beater or the like must be provided near the opening through which the treated material is supplied. A resulting problem is that particulates end up adhering to the mechanical dispersion mechanism, such as the attached disperser or beater.
Furthermore, since the supplied particulate travels along with the ascending air current formed in the straight pipe, lengthening the straight pipe is the only way to prolong the particulate residence time in the straight pipe and to improve the drying conditions (slowing the flow rate of the ascending air current limits the amount treated and the like), leading to an increase in the size of the equipment.
In addition, since the particulates travel along with the ascending air current in the straight pipe in such devices, often the same heated gas is in contact with the particulates as they travel. There is thus some concern that the heat exchange between the particulates and the heated gas as well as the evaporation of moisture from the particulates heated by this heat exchange reach a critical state soon after contact between the particulates and heated gas, with a dramatic decrease in the subsequent heat exchange and evaporation. A method that has been adopted to increase drying efficiency is to provide a curved part in the middle of the straight drying pipe, so as to abruptly alter the direction of flow in the curved part and thus produce instantaneous changes in the rate between the particulates and the accompanying heated gas, thereby exchanging the heated gas. However, in this case as well, the particulate residence time cannot be prolonged, and it is difficult to dry the particulate to below the specified moisture. A problem is thus that wet particulates adhere to the curved part provided in the middle of the drying pipe, in the same manner as when a disperser, beater, or the like is provided as described above.
Particulates adhering inside the device as described above hinder the operating stability of the apparatus. In addition, material adhering inside the device sometimes falls off as a result of frequent heat degeneration and thermal deterioration, becoming mixed in the form of impurities with the final product. Such contamination of the final product is a major issue.
The particulate drying action in the flash dryer depends solely on the quantity of heat of the heated gas, so attaching equipment to increase the amount of hot air, elevate the hot air temperature, and the like in order to enhance the extent of drying leads to the problems of greater size and higher running costs.
Fluidized bed dryers which are widely known as apparatuses for drying particulates using heated gas in a manner similar to that of flash dryers are apparatuses having a structure in which the container is divided into two upper and lower chambers by a perforated plate such as a wire netting, the upper chamber is filled with the particulates, and the heated gas is blown from the lower chamber through the perforated plate into the upper chamber, causing the particulates to fluidize and dry.
In such a fluidized bed dryer, the drying time can be set as desired, and the particulates can always be kept in contact with fresh heated gas, resulting in the advantage that the particulates can be dried to an extremely low moisture content. Another advantage is that such fluidized bed dryers allow the particulates to be uniformly dried.
However, when fluidized bed dryers are used in the case of particulates having a high moisture content, the particulate layer is difficult to fluidize, and the particulates are not adequately dispersed. As a result, dumpling-shaped lumps are produced in the final product, and there are also problems such as particulate adhesion on the machine walls.
Thus, when particulates having a high moisture content are dried, the particulates are generally first dried in a flash dryer such as that described above to a moisture that will not result in adhesion or lumps, and a fluidized bed dryer is then often used as a finishing dryer.
In view of the problems described above in conventional flash dryers and fluidized bed dryers in which particulates are dried by heated gas, an object of the present invention is to provide a particulate drying method and drying apparatus which can promote the dispersion of the particulates in the dryer and prolong the particulate residence time so as to improve the drying conditions while retaining the advantages of conventional flash dryers.